1. Field of the Invention
The present invention relates to a method for driving an AC-type plasma display panel (hereinafter referred to as an AC-PDP), more particularly, a surface-discharge type AC-PDP.
2. Description of the Background Art
As is well known, plasma display panels have two sheets of glass and small discharge cells (pixels) arranged therebetween, which are studied in various ways as thin-type television or display monitors. Known plasma display panels include AC-type plasma display panels (AC-PDPs) having a memory function. The AC-PDPs include surface-discharge type AC-PDPs. FIG. 38 is a perspective view showing the structure of a surface-discharge type AC-PDP. Japanese Patent Aplication No. 7-140922 and Japanese Patent Aplication No. 7-287548, for example, show surface-discharge type AC-PDPs having structure like this. In the diagram, the surface-discharge type plasma display panel 1 includes a front glass substrate 2 serving as a display face, a rear glass substrate 3 provided opposite to the front glass substrate 2 with a discharge space interposed therebetween, first row electrodes 4(X1-Xn) and second row electrodes 5(Y1-Yn) formed in pairs on the front glass substrate, a dielectric layer 6 covering the first and second row electrodes 4 and 5, an MgO (magnesium oxide) layer 7 formed on the dielectric layer 6 by deposition or the like, column electrodes 8(W1-Wm) formed perpendicular to the first and second row electrodes 4 and 5 on the rear glass substrate 3, phosphor layers 9 formed over the column electrodes 8 in order like stripes to emit red, green, and blue lights for the respective column electrodes 8, and partitions 10 formed between the column electrodes 8, 8 to separate the discharge cells and also to prevent the PDP from being broken by atmospheric pressure. The space between the glass substrates 2 and 3 is filled with a discharge gas, such as an Ne--Xe mixture gas or an He--Xe mixture gas, at a pressure not higher than atmospheric pressure. The discharge cells serving as pixels are formed at intersections of the pairs of the row electrodes 4, 5 and the column electrodes 8 perpendicular to the row electrodes 4, 5. Hereinafter the first row electrodes may be referred to as X electrodes, the second row electrodes as Y electrodes, and the column electrodes as W electrodes.
Next, its operation will be described. Voltage pulses are alternately applied between the first row electrodes 4 and the second row electrodes 5 to cause discharge with its polarity inverted for each half cycle to cause the cells to emit light. In color display, the phosphor layers 9 formed in the individual cells are excited by ultraviolet rays generated by the discharge and emit light. The first row electrodes 4 and the second row electrodes 5 that discharge for display are covered with the dielectric layer 6. Accordingly, once discharge occurs between the electrodes in cells, electrons and ions produced in the discharge space move in the direction of the applied voltage, and are accumulated on the dielectric layer 6. The charge of the electrons and ions accumulated on the dielectric layer are called wall charge. The electric field formed by the wall charge acts to weaken the applied electric field, and the discharge therefore rapidly disappears as the wall charge forms. After the discharge has disappeared, electric field having reverse polarity to that of the preceding discharge is applied, and then the electric field formed by wall charge and the applied electric field overlap, which allows discharge to occur at lower voltage than the preceding discharge. Subsequently, this lower voltage is inverted for each half cycle to sustain the discharge. The AC-PDP originally has this function, which is called a memory function. The discharge sustained at lower voltage with the aid of the memory function is called discharge sustain, and the voltage pulses applied to the first row electrodes 4 and the second row electrodes 5 for each half cycle are called sustain pulses. This discharge sustain lasts until the wall charge disappears, as long as the sustain pulses are applied. Eliminating the wall charge is called erasing and forming the wall charge on the dielectric layer in the first place is called writing.
Next, tonal display by the AC-PDP will be described briefly. FIG. 39 is a diagram showing the structure of one field in tonal display shown in Japanese Patent Aplication No. 7-160218, for example. One field is a time for output of one complete screen of picture, which is about 16.7 mS (60 Hz) in NTSC. In the drawing, the display lines correspond to the lines formed of the first and second row electrodes in the row direction in the AC-PDP. The lateral direction in the drawing shows passage of time. One field includes some subfields, and each of the subfields includes a reset period, an address period, and a discharge sustain period. For example, in display with 256 tones, one field includes eight subfields having respective discharge sustain periods with proportions of powers of 2, such as 1, 2, 4, 8, 16, 32, 64, 128. Although the entirety of one field shown in FIG. 39 is utilized as reset periods, address periods and discharge sustain periods, these periods may be uniformly distributed in one field with another type of periods, or may be compressed somewhere in one field.
FIG. 40 shows voltage waveforms in one subfield in the conventional method for driving a plasma display panel shown in Japanese Patent Aplication No. 7-160218, for example. In this conventional example, the first row electrodes X are connected in common, so that the same voltage is applied to all the first row electrodes X. The second row electrodes Y and the column electrodes W allow separate application of voltages to the individual lines. FIG. 40 shows voltage waveforms to a column electrode Wj, the first row electrodes X, and the second row electrodes Y1, Y2, Yn, from the top.
First, the reset period is a period for bringing all cells in the AC-type plasma display into the same state, in which an entire-face write pulse Pxp (priming pulse) is applied to the first row electrodes X connected in common in the entire screen at time "ta" at the beginning of the reset period in FIG. 40. This entire-face write pulse Pxp is set equal to or higher than the discharge starting voltage between the first row electrodes X and the second row electrodes Y so that all cells discharge and emit light independently of whether they emitted light or not in the preceding subfield. At this time, a voltage pulse is applied also to the column electrodes W to reduce potential difference between the X and W electrodes so that discharge will not readily occur between the first row electrodes X and the column electrodes W, which is approximately set at half of the voltage between the X and Y electrodes. It is not essential to apply this pulse. The application of the entire-face write pulse Pxp between the X and Y electrodes causes strong discharge between X and Y, so that a large amount of wall charge is accumulated between the X and Y electrodes, and the discharge ends. Next, at time "tb" in FIG. 40, the entire-face write pulse Pxp falls. When the voltage disappears between the first row electrodes X and the second row electrodes Y, the electric field by the wall charge accumulated by the entire-face write pulse Pxp is left between the X and Y electrodes. This electric field is so large as to start discharge by itself, and thus discharge takes place again between the X-Y electrodes. However, since no external voltage is applied, electrons and ions caused in this discharge are neutralized and disappear, without being attracted to the row electrodes X and Y. Thus, it is possible to bring all cells into a state with no wall charge for reset, by writing and erasing all cells independently of presence/absence of wall charge in the preceding subfield. The discharge caused by the accumulated wall charge without application of external voltage and erasing the wall charge is called self-erasing discharge.
When the reset period ends, at time "tc" in FIG. 40, almost no wall charge is left over the first row electrodes and second row electrodes. On the other hand, in the discharge cells, charged particles produced by the discharge with the previous entire-face write pulse Pxp are left. The charged particles, serving as priming for write discharge, ensure discharge in the following write operation. The entire-face write pulse Pxp may therefore be called a priming pulse. Accordingly, one pulse provides both of a priming effect and an erasing effect.
In the write address period, negative scanning pulses Scyp are applied to the independent second row electrodes Y1 to Yn in this order for scanning. The column electrodes W are supplied with positive address pulses Awp corresponding to contents of image data. The scanning pulses Scyp applied to the second row electrodes Y and the address pulses Awp applied to the column electrodes W allow matrix-selection of arbitrary cells in the screen. The total voltage of the scanning pulse Scyp and address pulse Awp is set not lower than the discharge starting voltage between the Y-W electrodes in cells, so that cells simultaneously supplied with the scanning pulse Scyp and the address pulse Awp discharge between the Y and W electrodes. The common first row electrodes X are kept at positive voltage during the address period. This voltage value is set so that it does not induce discharge between the X and Y electrodes even if it overlaps with the voltage value of the scanning pulse Scyp, but so that when discharge occurs between the Y and W electrodes, the discharge between the Y and W electrodes can trigger discharge between the X and Y electrodes at the same time. The discharge between the X and Y electrodes triggered by discharge between the Y and W electrodes may be called write discharge sustain. The write discharge sustain accumulates wall charge over the first and second row electrodes.
After the entire screen has been scanned, a sustain pulse Sp is applied to the entire screen all at once, so that only the cells having wall charge accumulated in the address period make discharge sustain. After discharge sustain has been produced for a given number of times, the next subfield starts. In the reset period, the entire-face write pulse Pxp is applied to all cells for reset. Thus, all cells are made to discharge at the beginning of each subfield and to accumulate wall charge, and then the wall charge in all cells are erased for reset by self-erasing discharge, so that the address write can always be performed in the same condition.
The driving method in which the write address periods and the discharge sustain periods are separated in the entire screen of an AC-type plasma display as described above is called an "address/display (sustain) separating method." Since the above-described entire-face write is performed in given cycles independently of display information, it reduces the contrast. For example, it causes the screen to be seen somewhat white in black display. It is not necessarily required that the entire-face write be performed for each subfield, since its priming effect lasts for a relatively long time. There are methods for improving the contrast by lighting the entire face for a reduced number of times for each field.
FIG. 41 is a diagram showing voltage waveforms in one subfield in the method for driving a plasma display described in Japanese Patent Aplication No. 8-278766. In the drawing, although the pulse Pxp applied in the reset period is set at discharge starting voltage between the first row electrodes X and the second row electrodes Y similarly to that shown in FIG. 40, its pulse width is as short as about 1 .mu.S. This driving method utilizes the characteristic of PDP that when a voltage pulse exceeding discharge starting voltage is applied to electrodes, the time from the moment the pulse rises to the moment discharge starts, or the discharge delay time, largely differs depending on presence/absence of the wall charge that acts while being superimposed upon Pxp. Although it depends on the cell structure and the kind of the entrapped gas, the discharge delay time is typically 0.1 to 0.6 .mu.S in the presence of wall charge, and is larger than 1.0 .mu.S in the absence of wall charge. When Pxp with a pulse width of 1 .mu.S is applied, it is possible to selectively reset only the cells that were lit in the preceding subfield.
With the use of this driving method, it is possible to entirely write/reset with Pxp having a larger pulse width shown in FIG. 40 in certain subfields in one field and selectively light/reset with Pxp having a smaller pulse width shown in FIG. 41 in remaining subfields, so as to reduce the number of times the entire screen is lit in one field, which suppresses luminance in black display.
In FIG. 41, the subfields are separated into subfields in which the entire face is written and subfields in which cells lit in the previous subfields are selectively lit by using pulses having such a high voltage as can induce discharge even in the absence of wall charge and controlling the pulse width. However, the subfields can be separated by setting the voltage value of Pxp so that the discharge starting voltage is exceeded only in the cells having wall charge. (Hereinafter, referred to as an erase pulse Exp in this case.) In this case, it may be called a small-width erase pulse or a large-width erase pulse, depending on the pulse width of Exp. Although the small-width erasing and large-width erasing are not described in detail herein since they are known to AC-PDP engineers, the contents are described in "Plasma Display" (Kenichi Owaki, et al. Kyoritsu Shuppan, 1983), for example. The small-width erase pulse has a voltage value around that of the sustain pulse and a pulse width of about 0.5 .mu.S. Application of this pulse erases wall charge since the pulse is interrupted in the progress of discharge, that is, before wall charge of reverse polarity forms.
Japanese Patent Aplication No. 7-49663 describes another method for suppressing brightness in black display. FIG. 42 shows the method for driving a plasma display described in Japanese Aplication No. 7-49663. One field includes a plurality of subfield groups each including one entire-face write/reset period, a plurality of discharge sustain periods with equal luminance level, and address write periods corresponding to the discharge sustain periods. In the drawing, SF1A, SF1B, and SF1C, SF2A, SF2B, and SF2C, and SF3A, SF3B, and SF3C form the respective independent subfield groups. FIG. 43 is a diagram showing light-emitting pattern in the subfield group including SF1A, SF1B, SF1C, among the subfield groups above. In the case of the relative luminance level 48, write address processing is performed in the first write period, and light is emitted in all of the periods SF1A, SF1B, SF1C. In the case of the relative luminance level of 32, write address processing is performed in the second write period and light is emitted in SF1B, SF1C. In the case of the relative luminance level 16, write address processing is performed in the third write period and light is emitted only in SF1C. It is thus possible to display a plurality of luminance tones with a single entire-face write/reset period and a plurality of write address periods. This allows reduction in the number of times the entire face is written/reset in one field, which suppresses luminance in black display.
In the conventional method for driving a plasma display, if it has 480 lines of row electrodes, for example, the electrode on the first line is not made to generate discharge sustain after it has been scanned for write address until the electrode on the 480th line has been scanned. Accordingly, only a limited time can be used for discharge sustain. Then, for causing discharge sustain an increased number of times, it is necessary to increase the frequency of the discharge sustain, or reduce the pulse width of signal in the write address periods, or to reduce the number of subfields. Increasing the frequency of discharge sustain reduces the luminous efficiency in discharge. Shortening the pulse width of signal in write address periods reduces the write margin, and then imperfect writing of address data will deteriorate the picture quality. Further, reducing the number of subfields reduces the number of displayable tones, reducing the display perfornance.
Further, with a plasma display panel with higher definition, the increased number of display lines require increased write time of address data. Then it is necessary to further increase the discharge sustain frequency, shorten the pulse width of signal in write address periods, or to reduce the number of subfields. In order to avoid reduction in the number of subfields, there are methods in which the column electrodes are divided into upper and lower halves, and address data is written for every two lines, so as to shorten the address periods. However, this complicates the structure of the panel. Moreover, this requires a driver IC for driving doubled column electrodes, which increases the cost of the panel.
If the number of times of the entire-face write/reset is extremely reduced to suppress luminance in black display, address data will be defectively written to considerably deteriorate the picture quality.
Further, for the purpose of suppressing luminance in black display, if the number of times of writing address is increased, the time for discharge sustain is reduced. Then, it is necessary to increase the discharge sustain frequency, or reduce the pulse width of signal in write address periods, or to reduce the number of subfields.
Further, as described above, a PDP makes tonal display with one field divided into a plurality of subfields with different weights of luminance information. In such a method for tonal display, the light-emitting timing in one field differs depending on the luminance level to be displayed. Accordingly, when moving picture, more specifically, picture with smoothly varying luminance level moves on the screen in the direction of change of the luminance level, bands like stripes are seen, which are not seen when the picture is standing still. This problem is called pseudo contouring of moving picture. The mechanism of occurrence of this problem is specifically explained in "Modern Technology of Plasma Display" (Shigeo Mikoshiba, ED Research, 1996), for example, which suggests that this phenomenon relates to evenness in time in the light-emitting pattern in one field. FIG. 44 shows light-emitting pattern in a conventional plasma display panel driving method. As shown in the drawing, the center of emission is located in different positions in one field between the relative luminance levels 128 and 127, causing lack of evenness in time in the light-emitting pattern.
FIG. 45 shows an example of moving picture, more specifically, a smoothly changing picture moving from left to right on the screen, in a conventional plasma display panel driving method. In the drawing, the horizontal axis shows the position on the screen and the vertical axis shows time. In the first field, the discharge cells A and B display the relative luminance level 128 and the discharge cells C to G display the relative luminance level 127. Subsequently, it moves to the right, one pixel for every two fields. In this case, the eyes of a man unconsciously fallow the moving picture, as shown by the broken lines. FIG. 45 can be rewritten as shown in FIG. 46 on the horizontal axis showing positions on the retina. Shown in the bottom in FIG. 46 is the amount of stimulus with respect to the position on the retina, where the relative luminance level 128 is sensed between a and "b" and the relative luminance level 127 is sensed between "c" and "d," with a less perceptible area existing between "b" and "c." This area is sensed as a pseudo contour of the moving picture.
It is known that the problem of pseudo contouring of moving picture can be lightened by compressing the luminance information in one field, or by dividing a subfield having the most heavily weighted luminance information and dispersing them in the field, for example. However, either of the methods above reduces time utilization efficiency in a field or increases the number of subfields, which necessitates shortening the pulse width of signal in address write periods or increasing the frequency of discharge sustain. Then, as stated before, the luminous efficiency in discharge is reduced or the margin is reduced, leading to deterioration of picture quality.